This invention relates generally to screws having a self-fastening feature. The purpose of a self-fastening feature in a screw is to prevent undesired loosening of a screw engaged in a workpiece due to vibration and other factors. In the art, the primary self-locking vehicle is a lockwasher positioned intermediate the head of the screw and the surface of the workpiece. The lockwasher may be provided with teeth of various configurations which engage in the surface of the workpiece to hold the screw.
The "teeth" on the lockwasher are usually deformable during engagement to provide a bias on the screw threads to increase the holding forces tending to hold the screw in position. Lockwashers have proved less than completely satisfactory because of the expense of separate parts and the use thereof and because of the possibility of some relative rotation between the lockwasher and the screw, the screw not being affirmatively held. A variation in the use of lockwashers is to capture a lockwasher on the body of the screw adjacent the head. The lockwasher is still displaceable relative to the screw and this arrangement has likewise not proved completely satisfactory.
In the Art, it has been proposed to form tabs in flanges projecting radially from the head of the screw to serve the lockwasher function. Thus, in U.S. Pat. No. 2,226,491, it is proposed to twist a dome-shaped flange (FIGS. 4-6) or a planar flange (FIGS. 7-9). In the former case the dome-shape of the tabs prevents proper distortion during use while the planar tab provides insufficient retaining force. In U.S. Pat. No. 3,343,580 is proposed to twist a planar flange so that the lowermost projections of the tabs define a sinusoidal envelope, an arrangement that does not provide maximum uniform force.
Neither of the foregoing arrangements have achieved acceptance in the market place and neither have proved completely satisfactory. By providing a screw having flanges integrally formed with the head and by forming the flanges as more particularly described below, the deficiencies of the prior art have been avoided and a new and highly effective self-locking screw is provided.